Both the investment casting process and the lost wax shell mold building process are well known, for example, as is apparent from the Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751. The lost wax shell-mold building process involves repeatedly dipping a wax or other fugitive pattern of the article to be cast in ceramic slurry that is contained in a dip pot to provide a ceramic slurry layer on the pattern, draining excess slurry, stuccoing the slurry with coarse ceramic particles to provide a stucco layer on the slurry layer, and drying individual or multiple stuccoed slurry layers to build up a shell mold of desired wall thickness on the pattern. The green shell mold/pattern assembly then is subjected to a pattern removal operation to selectively remove the pattern from the shell mold. Following pattern removal, the green shell mold is fired at elevated temperature to develop mold strength for casting of molten metal or alloy therein.
The ceramic slurry typically is contained in a dip pot having an open upper end so that the pattern can be dipped by robot or manually into the slurry during the shell mold building process. Multiple dip pots typically are provided with each dip pot containing a different ceramic slurry to be applied to the pattern in the shell mold building process.
A common ceramic slurry includes a mixture of ceramic flour (powder), a basic colloidal silica as a liquid binder, and other ingredients to provide an aqueous slurry. When such a ceramic slurry is exposed to ambient air, the slurry is observed to destabilize over time as evidenced by premature gelling and change in viscosity of the slurry binder over time. Such gelling and viscosity changes over time adversely affect the suitability of the ceramic slurry for use in making investment shell molds, shortening the working life of the ceramic slurry.